1. Field of the Invention
This invention generally relates to a support apparatus for a vehicle drive unit. More specifically, the present invention relates to a support apparatus that is especially useful for a vehicle drive unit of hybrid cars and electric cars.
2. Background Information
Vehicle drive units are supported in a variety of ways on a vehicle. Some drive units are supported on a sub-frame of the vehicle by a support apparatus, while some drive units are mounted directly to the body of the vehicle. With regard to a support apparatus for a rear differential unit, the rear differential unit has conventionally been supported to a body of a vehicle via a sub frame or suspension member to reduce pitching vibration from being transmitted to the body of the vehicle. A rear differential unit is a gear system that is typically employed in a rear-wheel drive vehicle. The rear differential transfers power from a drive or propeller shaft to output axle shafts, which are splined to a differential case at a right angle. When a drive shaft vibrates as the vehicle accelerates, the rear differential unit is subjected to a pitching vibration, which is a rotational vibration on the front and rear sides of the vehicle about a center of the gravity.
Alternatively, there have been known support structures for rear differential units, such as the one described in Japanese Laid-Open Patent Publication H08-132900, which do not utilize a sub-frame. In this publication, a rear differential unit is supported by front and rear cross members, which are mounted directly to a floor panel. In this structure, the rear differential unit is supported by the front cross member at a front support point, and by the rear cross member at two points, rear support points. These points are positioned such that the pitching vibration will be canceled on front and rear sides of the axle center due to an overlap of resonant vibrations having different phases. In other words, where the distance between the axle centerline and the front support point is L1, and the distance between the axle centerline and the rear support points along a rear drive shaft is L2, a ratio of the distance L1 to the distance L2 is 2:1. Due to different phases of the resonant vibrations, this ratio allows the pitching vibrations transmitted to rear differential unit to cancel each other on front and rear sides of the axle centerline. Accordingly, since pitching resonance transmitted to the rear differential unit will be phase-canceled, the pitching will not typically be transmitted to the body of the vehicle.
Since the conventional sub-frame can be eliminated with this structure, the weight and cost of the vehicle can be reduced. Also, the floor can be lowered without having to increase vibrations and noises transmitted to the interior of the vehicle.
However, such conventional support structure for a rear differential unit does not go beyond establishing a ratio between dimensions of the front and rear portions of the rear drive shaft as divided by the axle centerline. In other words, this structure is only effective in a two-shafts structure, where the input and output shafts are in a crosswise arrangement and torque is inputted from one of the shafts and outputted to the other, as is the case in a rear differential unit. Thus, this vibration suppression method cannot be used in a single shaft arrangement.
Moreover, when this conventional structure is applied to a drive unit utilizing an electric motor, instead of a rear differential unit, this vibration suppression does not function. A drive unit for an electric vehicle includes a motor, a reduction gear and a differential unit. Since drive units are well known in the art, the structure and function of a drive unit would be obvious to one of ordinary skill in the art without further explanation herein.
Since there is only an output shaft, instead of two shafts, in a drive unit, the suppression of pitching vibration by way of phase-cancellation of pitching resonance is not effective. Therefore, the conventional support structure is not sufficiently effective in a single drive shaft. In other words, the phase-canceling effect obtained in a 2-shafts structure, which reduces pitching vibrations, can not be obtained in a single shaft structure.
Furthermore, a drive unit for a hybrid or electric car includes electric motors. The vehicle drive unit includes a reduction gear when one is provided. Hybrid vehicles are vehicles that run on both electrical energy and mechanical energy, while electric cars run on electrical energy. Typically, the electric motors are operatively connected to driving wheels through a power transmission mechanism of the vehicle. While the vehicle is running, the electric motor normally drives the vehicle as power is supplied from a battery. When the vehicle is in a decelerating operation, the electric motor functions as an electric generator to generate deceleration recovery power, with which the battery is charged. In other words, in a hybrid car, the electric motors are also source of torque. Therefore, it is difficult to reduce vibrations with a conventional support structure.
Besides, when a vehicle is operated by motors, the motors create a large torque and a high rotational moment about the drive shaft at a low rotational speed. Accordingly, it is preferable that the rotational rigidity of the support apparatus be high. However, if the translational rigidity is too low, too much vibration will be transmitted to the vehicle body.
Due to the reasons set forth above, the conventional support structure does not provide sufficient vibration suppression function to be employed in a drive unit for a hybrid or electric car. Conventional support structures particularly cannot reduce vibrations with small displacements, such as jerking vibrations resulting from reaction forces to driving force that occurs when the vehicle starts moving.
In view of the above, there exists a need for a support apparatus for a vehicle drive unit which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
The object of the present invention is to provide a support apparatus for drive unit having an electric motor, for use in a hybrid vehicle and/or electric car. The support apparatus of the present invention provides low rigidity in the translational direction and high rigidity against moment inputted by the drive shaft at the center of gravity. In this manner the support apparatus can both reduce vibration due to the low rigidity in the translational direction, and reduce jerking vibration at the start-up of the vehicle due to the high rigidity against moment transmitted from the drive shaft.
The foregoing objects can basically be attained by providing a support apparatus for a vehicle drive unit. The vehicle drive unit has a center of gravity with an inertial coordinate system defined by three inertial axes orthogonally intersecting at a single point that coincides with the center of gravity. The support apparatus comprises a vehicle support member and at least three elastic mounting members coupled to the vehicle support member with individual elastic centers of the elastic mounting members lying within a mounting plane. The elastic mounting members define an entire elastic support system having an elastic coordinate system with three elastic axes orthogonally intersecting at a single point. The single intersection point coincides with an overall elastic center of the elastic mounting members. The elastic mounting members are further dimensioned and arranged on the vehicle support member so that the elastic mounting members are adapted to support two of the inertial axes of the drive unit within the mounting plane of the elastic mounting members.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.